Using calcium carbide slag as raw material and isocaprylic acid as the calcium extraction agent, nano calcium carbonate was prepared through CO₂ carbonization and characterized. Under the conditions of 80% saponification rate, 80 ℃ saponification temperature, 40 min saponification time, 80 ℃decomposition temperature, 60 min decomposition time, 0.5 mol/L calcium chloride concentration, and the volume ratio of the oil phase to the water phase is 2.0∶1.0, the extraction rate of calcium ions from calcium carbide slag reached 96.40%. Nano calcium carbonate was then prepared using the gas?liquid carbonization method. The optimal carbonization efficiency was achieved at a reaction temperature of 6 ℃, stirring speed of 400 r/min, liquid droplet rate of 35 mL/min, and CO₂ flow rate of 100 mL/min. Characterization by X?ray fluorescence spectroscopy, X?ray diffraction, and scanning electron microscopy showed that the nano calcium carbonate product was of the calcite type, with a particle size of 78.8 nm, calcium content was 94.10%, and uniform particle size. This study optimizes the process of producing nano calcium carbonate from calcium carbide slag, enabling high?value reuse of solid waste and CO₂ emission reduction, providing new insights for the preparation of nano calcium carbonate products.